ISO1050: Low resistance failure between CANHI, CANLO, GND

Hi Ryan,

Thank you for using TI E2E.

• The 100ohm resistancereading on the CAN lines does seem low.
  • On a good unit, the differential impedance between CANH and CANL should be between 30k and 80k-ohms; 
  • The single ended impedance reading should be ~half of this.
• Curve tracing the CANH and CANL pins w.r.t GND2  would be a good predictor of any potential EOS/damage inside the chip.  If possible, these pins could be curve traced to +/- 25V with a current limit of  <50mA.   This will help us look for signs of early/premature turn-on (if any) of the components inside
• On the second unit you mentioned, it looks like this is constrained to just CANH. So the sanity check experiments proposed above may further confirm this as a trend or provide a different data point. It will be helpful either way.
• Supply values: voltage values look healthy and should be fine. Do you see any abnormally high current consumption on either side 1 or side 2 of the chip? (when compared to a good one)
• Power -up default value on CANH, CANL: Typically, when provided with a 60ohm termination resistor between CANH and CANL, and with TXD input = high; the default value on the CAN lines must be CANH=CANL = Vcc2/2 = ~2.525V  [ = Recessive] in your case. If one or both of these pins are off, then that will offer additional clues on what may have gone wrong.
• Could you please tell us which ESD diodesare being used? PESD1CAN and PESD2CAN are commonly used, but I wish to double -check and not assume anything at this point. 
  • The key thing here is to position the TVS/ESD diode breakdown voltage. 
  • It must be above the Operational range of the part (+/- 12V per the data sheet) so as to not interrupt normal operation, and also must be below the Abs Max value ( = damage rating) of the part, so that the chip is not damaged.
  • When positioned this way, normal operation is uninterrupted. But when an ESD or other high voltage event happens, then the diode will trigger first and still protect the device. 
  • If the diode breakdown voltage is too high, then the ISO1050 may trigger first - which we don't want to happen.

• Choke details would also help as a sanity check. Chokes with inductances between 11uH - 100uH are common.

Possible cause(s): Still early to say for sure, but one of the CAN pins may have been damaged due to an EOS event or during handling/processing. Admittedly it is hard to figure out which system EOS event may have caused this.  This also depends on which ESD/TVS diode is chosen. We are not aware of any issues that might be caused or made worse by the potting process per se, but need to check. The curve trace data will help on this for sure.

Sorry for the broad list. Please feel free to let us know what you think and if it's possible to look into the above as time permits. We can also do a schematic review for you if you'd like - although that would be best done over email or private message(PM) to keep it off a public forum. We can then circle back and summarize the key points here later as needed.

We hope this helps. 

Best regards,

Abhi

Abhi,

Thank you for your in depth and well considered reply.

The tests you have outlined above I will perform and provide results within the next 2 weeks.
I am happy to provide information w.r.t. the choke and the ESD diodes used over email.

I am of the same opinion that it is likely caused by an EOS event - it would be nice to identify this and be able to put in place appropriate protection to prevent this in the future.

I have also taken the time to create a basic ESD simulator, to observe the effects seen by the ISO1050 to see if this provides any insight.

I would like to take you up on the offer of a schematic review if possible please - I have sent you a request on here with my email address.

Best Regards,
Ryan

Hi Ryan,

You are welcome, the plan sounds good.
Yes, we can definitely review components (chokes, diodes) and schematic over PM/email and then post a summary here later.

Thanks and regards,
Abhi